If the common app gives you a maximum amount of words allowed, should you aim use it all or does that not really matter?
The common app word limit is already pretty small so I would aim to fulfill it. But if you can’t and you feel like you’ve said everything, definitely don’t write more just for the sake of filling it up.
As a general rule, being 5-10% under the word count is tolerable. Any shorter than this and it starts becoming a little bizarre.